The present invention relates to casting methods and apparatus and, in particular, to evaporative pattern casting methods and apparatus. More particularly, the present insertion of an evaporative pattern into a casting flask or pouring container, packing an unbound molding medium around the pattern in the flask, pouring molten metal into the flask to displace the pattern, treating the resulting gases from the evaporation of the pattern, removing the formed and cooled casting, and recycling the molding medium for further casting.
The evaporative pattern casting process, also known as the lost foam process or the full mold process, is a process for making metal castings in which a pattern made of a material which evaporates or gasifies upon contact with molten metal is inserted into a pouring flask or container and surrounded by a molding medium. The pattern is typically made of a plastic foam material, e.g., polystyrene, and is substantially the same size and shape as the casting to be produced, i.e., it is a positive pattern. The evaporative pattern casting process, whether the pattern is made individually or in mass quantities, is similar in principle in some respects to the lost wax process or precision casting process which, also, has heretofore been used in foundries. A unique feature of the evaporative pattern casting process, however, lies initially in its suggestion that the plastic pattern material can be evaporated by the molten metal itself rather than by the application of external heat before the metal is actually poured. In the evaporative pattern casting process as originally conceived, the pattern was embedded in a molding medium comprising bonded granular particles. See Shroyer U.S. Pat. No. 2,830,343. A second development in the art was the use of dry, unbonded granular molding medium. See Smith U.S. Pat. No. 3,157,924. The evaporative pattern casting process has shown that, in principle, castings can be produced in dry unbonded sand or granular molding material. A third development involves the elimination of a refractory wash coat which normally has been required to be applied to the evaporative pattern. See applicant's U.S. Pat. No. 4,651,798.
Throughout the years, there have been a number of improvements made in the evaporative pattern casting process. For example, a number of patents have been obtained in the evaporative pattern casting art. U.S. Pat. No. 4,593,739 to Van Rens et al. describes a method and apparatus for packing sand around a mold pattern by vibration. U.S. Pat. No. 4,600,046 to Bailey et al. describes a molding apparatus and process including a sand compaction system. A rigid mold flask is adapted to contain a mold pattern and sand. A support is provided for resiliently supporting the mold flask for horizontal movement only of the mold flask as a whole body. U.S. Pat. No. 4,685,504 to Bond et al. discloses a foundry sand feeding apparatus for use in feeding sand to an evaporative pattern casting mold. U.S. Pat. No. 4,454,906 to Musschoot discloses a vibratory method for packing foundry sand into a pattern prior to the pouring of molten metal. In the apparatus according to the '906 patent, the mold flask containing the pattern and sand is first vibrated at a frequency and stroke to produce accelerations in excess of the acceleration due to gravity to cause the sand to penetrate and fill the cavities. The accelerations then are reduced to produce an acceleration less than the acceleration due to gravity to compact the sand in place.
U.S. Pat. Nos. 3,581,802 and 3,678,989 to Krzyzanowski disclose a method and apparatus for making castings in which a body of granular material is confined in a container and a stream of gaseous fluid is introduced into the bottom of the container to impart to the granular material a state at least approaching fluidization. Into the thus agitated granular material a form is introduced consisting of material which is subject to consumption on contact with the molten metal. Gaseous fluid is also used after casting to agitate the granular material for facilitating removal of the finished casting therefrom. U.S. Pat. No. 3,861,447 to Hondo discloses a molding method in which negative or subatmospheric pressure is induced in the filler material in a flask containing an evaporative pattern. U.S. Pat. No. 4,690,201 to Van Rens discloses a lost foam mold pattern and associated method which facilitates suspension of the foam mold pattern assembly. U.S. Pat. No. 4,085,790 to Wittmoser discloses a casting method using a cavityless mold. At least two of the walls of the casting mold are formed by flexible membranes exposed to the atmosphere for creating a more a less isobaric pressure upon the application of suction to the interior of the casting mold. U.S. Pat. No. 4,139,045 to Vitt discloses a casting method and apparatus wherein the gas generated upon evaporation of the pattern is used to create an elevated pressure and a molten metal fills the space initially occupied by the pattern in the presence of the elevated pressure which is greater than atmospheric pressure.
U.S. Pat. No. 4,693,292 to Campbell discloses a method of casting metal articles by feeding molten metal upwardly against the force of gravity from a source of molten metal into a mold cavity where the metal is permitted to solidify within the cavity after which the metal feed is interrupted and the casting is removed from the cavity. U.S. Pat. No. 3,766,969 to Mezby et al. discloses an air breathing flask for a foundry mold. The flask has air breathing walls that are provided with a permeable layer, slotted vents, or screen vents in order to allow gases to escape from the vaporizable pattern. U.S. Pat. No. 3,572,421 to Mesey, et al., discloses a flask and pouring apparatus for casting metals using vaporizable patterns wherein the flask has air breathing walls.
Other patents, relating generally to the casting art, have attempted to recycle foundry byproducts or render them safe. U.S. Pat. No. 4,544,013 to Kearney et al., discloses a method of reclaiming sand used in a method of casting a metal. The method comprises blowing, after a predetermined lapse of time after the pouring of the metal into the mold, a combustion supporting gas into and through substantially the entire volume of the mold to levitate at least a region of the sand adjacent to the casting and to combust volatilized byproducts of the pattern entering the mold, and continuing the blowing of the combustion supporting gas to continue combustion and driving of the gas and byproducts of the combustion out of the mold. U.S. Pat. No. 4,291,739 to Baur discloses a method of manufacturing a hollow casting mold. The method includes the step of forming a casting mold body of a flowable binder-free backfilling material, stabilizing the casting mold body by negative pressure, and providing in the body a lost form of synthetic plastic material, particularly foamed synthetic plastic material. A vacuum pump operates during the casting process to carry away the products of combustion of the foam synthetic plastic material. U.S. Pat. No. 4,408,985 to Anderson et al. discloses a method of fixing hazardous substances in waste foundry sand. Certain dangerous substances are fixed into recyclable waste foundry sand so that they are less likely to leach out into the environment. This method uses a roasting process which causes dangerous substances to form insoluble compounds with the sand. An example is shown where hazardous waste foundry sand which contains acetic acid soluble lead is treated by the method.
U.S. Pat. No. 4,436,138 to Kondo discloses a method and apparatus for reclaiming molding sand. Centrifugal apparatus for removing impurities from the sand by abrasion and entrainment is disclosed. U.S. Pat. No. 4,620,586 to Musschoot discloses a method and apparatus for reclaiming foundry sand. A mold flask containing foundry sand shaped by a pattern to form a cavity into which molten metal has been poured is, after the metal has set, subjected to greatly reduced atmospheric pressure whereby to cause the moisture in the foundry sand to evaporate into water vapor, thereby removing moisture and heat from the sand so that the particles can be reused free of lumps or clumping of sand. U.S. Pat. No. 4,681,267 to Leidel et al. discloses a method of regenerating old casting sand, the sand being subject to rapid heating and extreme turbulence and crushing. U.S. Pat. No. 4,685,973 to Ashton discloses a method of reclaiming foundry sand wherein the foundry sand has been bonded using a sodium silicate binder. U.S. Pat. No. 4,354,641 to Smith discloses an apparatus for removing no-bake coatings from foundry sand and classifying the reclaimed sand. Foundry sand having nobake rigid coatings is discharged against a target to fracture the coatings from the sand grains on impact. U.S. Pat. Nos. 4,203,777 and 4,089,081 to Jacob disclose an apparatus and method for purifying particulate mold material. The sand is passed through layers of steel wool. U.S. Pat. No. 4,149,581 to Adkison et al. discloses a fine particle recycling method and apparatus. Fine particles of molding material are removed as a wet waste and subsequently delivered to a muller for mixing with sand and water in preparing molding material. U.S. Pat. No. 4,130,436 to Hauser et al. discloses a process for reclaiming foundry sand wastes using incremental heating followed by cooling. U.S. Pat. No. 4,113,510 to Richard discloses a process for regenerating foundry sand wherein resin in binder material accumulated around each of the particles is removed. U.S. Pat. No. 3,461,941 to Schumacher discloses a method of handling and cooling foundry sand. Unused sand is mixed with used sand from the breaking up of molds in an effort to reduce the need for cooling apparatus.
Other patents have been obtained for reducing pollution in the foundry industry. For example, U.S. Pat. No. 4,457,352 to Scheffer discloses a system and process for the abatement of casting pollution, reclaiming resin bonded sand, and/or recovering a low BTU fuel from castings. U.S. Pat. No. 4,478,572 to Selli discloses a plant and method for regenerating sand from foundry cores and molds by calcination in a fluidized bed furnace. U.S. Pat. No. 3,646,987 to Schumacher discloses a method for reducing dust pollution in foundries, and U.S. Pat. No. 3,838,732 to Overmyer discloses a contaminant collection system for a shaker table for removing mold sand from metal castings. These patents show especially the problems encountered in attempting to recycle the resin bonded sand used in the prior art techniques such as the green sand process.
Other patents of interest include U.S. Pat. No. 4,609,028 to Van Rens disclosing an evaporative pattern assembly for use in sand casting and U.S. Pat. No. 4,612,968 to Ashton et al. for a method of casting using expendable patterns wherein the pattern has a gas permeable refractory coating thereon. The particulate material is compacted to a maximum bulk density and a vacuum is applied to a compacted particulate material so as to create a sufficient pressure gradient in the compacted material to maintain the integrity of the gas permeable refractory coating. U.S. Pat. No. 4,616,689 to Denis discloses a foundry molding process and mold using a pattern of gasifiable material surrounded by sand free of a binding agent for low pressure precision casting. Molded metal parts are produced by forcing molten metal under low pressure upwardly into a mold chamber. Other patents of interest include U.S. Pat. Nos. 4,633,929 to Santangelo et al., 4,694,879 to Feuring, 4,509,579 to Pirrallo, 4,448,235 to Bishop, 4,482,000 to Reuter, 4,632,169 to Osborn et al., 4,640,333 to Martin et al., 4,657,063 to Morris, 4,673,023 to Winston, 4,691,754 to Trumbauer et al., 4,243,093 to Nieman, 4,640,728 to Martin et al., 3,868,986 to Olsen, 4,240,492 to Edwards et al., 3,496,989 to Paoli, 3,374,827 to Schebler, 3,374,824 to Snelling, 3,498,365 to Wittmoser, 3,889,737 to Olsen and 4,068,704 to Wittmoser.
U.S. Pat. No. 3,314,116 to Wittmoser et al. discloses the use of a refractory coating on an evaporative pattern in the evaporative pattern casting process. The use of a refractory coating is disadvantageous and is sought to be eliminated by the present invention. Such refractory coatings negatively affect proper gas evacuation and can result in casting imperfections.
Although substantial work has been done in the development of the evaporative pattern casting process, the technology still has not yet achieved its full capabilities. The automobile manufacturers have applied the technology for limited applications and in test cases. However, the technology has not heretofore been developed to the point where older, more well known casting processes, such as the green sand process, have been superceded. See, e.g., Modern Casting, Aug. 1981, pp. 36-37.
Nonetheless, the evaporative pattern casting process offers promise of substantial benefits, especially if an unbonded molding material is applied around the evaporative pattern. As originally conceived, see e.g., the Shroyer patent referred to above, the molding medium disposed around the evaporative pattern was bound, i.e., a glue binder material was used to hold the grains of the molding medium together, thus forming, in effect, a negative pattern around the positive pattern when the binder set. This was disadvantageous due to the high costs of molding medium binder treatment and subsequent molding medium cleaning and recovery. The later Smith 3,157,924 patent established that the molding medium could be unbound, but the use of an unbound molding medium has also not been without problems. Chiefly, these problems have been in the area of packing the molding medium into all the internal cavities and corners of the pattern, in preventing deformation of the pattern by the weight of the packed molding medium around the pattern, in preventing shifting of the pattern and molding medium when the metal is poured, and in providing a sufficiently "breathable" molding medium so as to allow escape of the gases generated when the pattern evaporates without disturbing the medium. A molding medium which has recently became available and which offers advantages to the evaporative pattern casting process is described in U.S. Pat. No. 4,651,798, issued to Leslie D. Rikker, the applicant herein.
Furthermore, the evaporative pattern casting process has heretofore had additional problems. For example, the gases generated by the evaporation of the pattern are pollutants which require treatment either to make them harmless or recycle them into useful products. The molding medium itself will become polluted by the escaping gases and provision must be made to clean the medium if it is to be recycled. Recycling is, of course, warranted and beneficial in view of the declining availability of the world's natural resources.
Large amounts of molding medium are required to be packed around the evaporative pattern and, accordingly, recycling of the molding medium offers significant advantages. Needless to say, the known green sand molding technique, for example, does not readily offer the same advantages insofar as recycling is concerned, because that technique employs a binding medium to bond the grains of the molding medium together, making it much more difficult to recycle the molding medium.
For example, when a binder is used it has to be reactivated by water and expensive reconditioning techniques must be applied in order to allow for recycling. Recycling is made even more difficult by cores produced from chemically bonded sand which cannot be easily separated out from the mold after the casting has been poured. These chemically bonded sand grains become mixed with the green sand creating additional reconditioning problems and expensive reclamation and disposal problems for the excess sand that has to be reused. Even more difficult is the recycling of the mold material made from so-called no-bake sand. This type of sand is usually coated with a resin composed of furan, phenolic, or urethane based polymers which, when catalyzed, cause the hardening of the resin. This renders the used sand unreuseable without the extensive grinding up of the molded sand into its original granular structure and the cleaning of the surface of the sand grains so that they will accept new coats of the binding resin.
Furthermore, the prior art techniques do not offer the production advantages which the evaporative pattern casting process, in principle, can enjoy. For example, because the evaporative pattern casting process can use an unbonded molding medium which is merely packed around the evaporative pattern, there is no need for substantial waiting periods for the molding medium to set. Furthermore, there is no need to remove a positive pattern from which a negative pattern is formed prior to metal pouring. There is, of course, no need even to produce a negative mold separate from the positive pattern in advance of metal pouring since the packing of the molding medium around the evaporative pattern accomplishes this objective in one step.
As previously explained, the ability to recycle offers significant production advantages to the evaporative pattern casting method. Also significant is the improved quality of castings which is, in principle, obtainable using the evaporative pattern casting technique because of the elimination of joint and flash lines, the possibility of eliminating the "wash" coat in the present invention which has, in the past, been utilized in the evaporative pattern casting technique between the pattern and the molding medium, and the inherent precision which the process offers due to the fact that the pattern is substantially an exact rendition, in foam, of the casting to be produced. Thus, the evaporative pattern casting process offers significant production advantages so that its application to large quantity casting will not be without substantial rewards, assuming the problems so far encountered in applying the technology can be overcome.